When opening new Internet Explorer window, a new browser session is issued. Therefore you can login with two different accounts on the same service (let's say gmail). The same does not apply when opening a new tab in the same window on IE.
How can I get a new browser session when opening a new tab or window on Firefox?
How about Google Chrome?

-nomergeoption to the command line, e.g. of a shortcut. – Martha Mar 24 '10 at 18:14